1. Field of the Invention
The present invention relates to an air ring plenum which surrounds a tubular film as it is extruded from a tubular die, the plenum directing air towards the film to cool the same prior to its blown expansion.
2. Discussion of the Prior Art
An annular air plenum typically used in the above-described environment is shown in FIG. 1. It is formed of 1/8" aluminum sheet which is bent and welded into a desired annular configuration. At least two air inlets 11, 13 are provided and the inner periphery 15 of the plenum has an annular outlet nozzle for directing an air flow towards the outer surface of an extruded tubular film which passes through the central opening 16. Air from the inlets 11 and 13 is directed counterclockwise in an outer annular chamber 17, which has a linearly decreasing cross-sectional shape, from inlets 11, 13 to points farthest removed therefrom. Air within outer chamber 17 passes radially through an annular screen 19, having perforations therein, into an inner annular chamber 21, which in turn guides the air radially to the outlet nozzle at the inner periphery 15 of the plenum.
Although, when precisely constructed, this air plenum provides good uniformity in air distribution about the outlet nozzle, it is difficult in practice to achieve a precise construction. This is because it is difficult to fabricate the aluminum with precision, particularly since it warps when welded. In addition, the use of a plurality of air inlets 11, 13, which serve the useful purpose of reducing variations in air flow from its entry into the plenum to various positions therearound, requires that all feed hoses supplying air to the inlets 11, 13 have the same air flow, which in practice is difficult to achieve, as the hoses typically have different lengths. Moreover, the perforated screen 19, which redirects the air flow radially inward, should have a relatively large length-to-diameter ratio for the holes therein to ensure proper radial orientation of the air flow, but the perforated screen, which consists of punched holes in an aluminum plate, typically has a length-to-diameter ratio of no greater than 1:1.
Furthermore, in order to ensure uniform air distribution around the plenum, it is necessary to have a linearly decreasing cross-sectional area. This is accomplished by fabricating an outer wall 23 of the plenum in a spiral pattern, which is a mathematically generated curve. For ease of construction this is sometimes approximated by using a circular shape offset from center, which introduces some inaccuracy in a desired air flow pattern.
Because of the difficulty of constructing the FIG. 1 plenum with precision, its construction cost is high.